The present invention relates to an optical modulator which permits ultrahigh-speed modulation.
FIG. 6 shows a conventional optical modulator. On an n-side electrode 10 there are laminated an n-type InP substrate 1, an n-type InP clad layer 2, an InGaAsP optical modulation waveguide 3, a non-doped InP clad layer 4, a p-type InP clad layer 5 and a p-type InGaAsP contact layer 6. The layers 3, 4, 5 and 6 are formed into a mesa, which is flanked by semi-insulating InP layers 7 in such a manner as to bury therein these layers. Protective silicon nitride films 8 cover the semi-insulating InP layers 7, and a p-side electrode 9 is formed in contact with the InGaAsP contact layer 6. A pad 11 is formed on the silicon nitride film 8 in a manner to be electrically connected to the p-side electrode 9.
With the optical modulator shown, it is possible to modulate incident light to the InGaAsP optical modulation waveguide layer 3 by applying a minus voltage to the p-side electrode 9 and a plus voltage to the n-side electrode 10. For example, in a case where the optical modulator is designed so that the photon energy of incident light thereto becomes smaller than the band gap energy of the InGaAsP optical modulation waveguide layer 3 by about 30 to about 60 meV, the incident light is hardly absorbed by the InGaAsP optical modulation wave-guide layer 3 and passes intact therethrough when no voltage is applied, but when voltage is applied, the incident light is mostly absorbed. By designing the modulator so that the photon energy of the incident light becomes sufficiently smaller than the band gap energy of the InGaAsP optical modulation waveguide layer 3, it is possible to modulate the incident light while keeping its intensity constant.
In the illustrated optical modulator, the capacitance of the pad 11 is 0.37 pF in a case where the semi-insulating InP layers 7, the silicon nitride film 8 and the pad 11 are, for instance, 2.25 .mu.m thick, 0.1 .mu.m thick and 100 .mu.m.phi. thick, respectively. An optimum design of the element for its operating voltage and the insertion loss of light is such as follows: The InGaAsP optical modulation waveguide layer 3 is about 0.25 .mu.m thick and about 2.5 .mu.m wide, the non-doped InP clad layer 4 is about 0.05 .mu.m thick, and the device is about 100 .mu.m thick and 200 to 300 .mu.m long. According to calculations by the present inventors, the capacitance between the p-side and n-side electrodes 9 and 10, except the capacitance of the pad 11, was 0.27 pF every 100 .mu.m of the device length. Consequently, the capacitance of the device is 0.91 pF to 1.18 pF. In case of constituting the system with 50 .OMEGA. for impedance matching of a high-frequency circuit, the cut-off frequency will be lower than 10 GHz. To make the cut-off frequency higher than 10 GHz, the capacitance of the optical modulator needs to be decreased and it is well-known that this can be done by decreasing the length of the device.
Present semiconductor device manufacturing techniques have difficulty in the cleavage of both ends of an optical modulator which has a length substantially equal to or smaller than the thickness of the device itself. Therefore, in the fabrication of an optical modulator which has a thickness of around 100 .mu.m as in the above example and a length of about 100 .mu.m or less, the cleavage of its both end faces is difficult and the cut-off frequency of the optical modulator cannot be made higher than 10 GHz by the known method of reducing the length of the device.
Moreover, pn junctions at input and output end faces of the conventional optical modulator are exposed to air, and hence water and oxygen in the air enter into the pn junctions, adversely affecting the durability and reliability of the optical modulator.